Lightroom: LR5.2 "nil value"

I have recently started a Trial of Lightroom 5.2 and until yesterday was very happy with the ease of use and the range of functions. When I open LR now I cannot access any of my images. If I try via the Develop module, a message appears saying "nil value". Yesterday I was ready to publish some photographs taken during a photography workshop to my Zenfolio website. I found I could not upload file sizes of between 9-15MB. This may be due to having to connect to the internet via an O2 dongle (HSPA). So I resized my shots and uploaded them manually from my ext. HDD. But I do need to be able to upload larger file sizes, ready for clients to select for printing via One Vision Imaging. Can you explain what I need to do to access my photos? They are backed up.